The present application relates to semiconductor packages.
Not to long ago a 30V synchronous FET die would have been approximately 15 mm squared for a given performance. The same performance can probably be matched by a die of approximately 9 mm squared or less with the available technology today. For electrical and thermal performance the most efficient packaging solution is a direct connection (using solder or the like) of a large cross-sectional area to an external pad of, for example, a circuit board. However, problems may arise as the die is reduced in size. For example, there will be design constraints based on what is possible to achieve in terms of distances between pads to prevent shorting, which may mean that large cross-sectional area joints are not always achievable.
In addition, the move by the industry to the use of lead-free assembly processes has resulted in detrimental residuals when assembling a die on a PCB using a lead-free solder. The gate and the source side of a semiconductor die are the areas most sensitive to contamination by lead-free solders.
Moreover, as the die gets thinner it becomes more and more difficult to solder a die directly to a PCB and yield a reliable assembly. This is especially true in applications where large die are used, such as IGBTs for automotive applications in hybrid and electric cars.
Not only is there a question of mechanical damage to the die by soldering the die directly to the PCB, there are questions over metalisations as well. Before the switch to lead-free assembly, there was ample data on how a die would react to being soldered. Data regarding the use of lead-free solder is not yet as available. For example, while it is known that the alloys being used for lead-free PCB assembly increase the consumption of the metalisation during soldering to levels not seen before, little is known about long term consumption.
Furthermore, tracking distances imposed by higher voltages pose some very different problems for a package. Thus, there is a necessity to separate the connections of dissimilar polarities by a large enough distance to meet the surface tracking distances given by standards such as UL840, which causes real issues for packages based on a fixed framework such as a can or a lead frame.